In medium or high voltage encapsulated switchgears, the electrical active part is arranged in a gas-tight housing, which defines an insulating space, said insulating space usually comprising an insulation gas and separating the housing from the electrical active part without letting electrical current to pass through. Thus, metal-encapsulated switchgears allow for a much more space-saving construction than switchgears which are insulated solely by ambient air.
For conventional encapsulated switchgears, insulation gases comprising a dielectric compound having a boiling point below −25° C. are used in order to prevent condensation over the complete operation temperature range. The required pressure of the insulation gas and/or the amount of the dielectric compound comprised in the insulation gas is governed by gas pressure measurement (with or without temperature compensation) or direct density measurement.
The equipment used for gas pressure measurement is in general relatively complex and expensive.
In addition, it is usually required that the insulation gas has a slight overpressure, which in medium voltage switchgears ranges typically from about 100 mbar to about 500 mbar, in order to allow for a precise pressure measurement in the insulating space of the switchgear. Due to this overpressure, the housing of the switchgear can be subject to mechanical stress and therefore be prone to gas leakage if appropriate technical measures are not taken.
However, the demands on the gas-tightness of the currently used switchgears are very strict, because conventional insulation gases with a high insulation and arc extinction performance have some environmental impact when released to the atmosphere and, in particular, have a relatively high global warming potential (GWP).
For this reason, the housing of the switchgear must be very robust even under the overpressure conditions mentioned above.
Also, for allowing repair work to be carried out in the inside of the housing, means are required for evacuating the housing prior to it being opened and reintroducing the insulation gas afterwards, before operation of the switchgear can be restarted.
The construction of the housing of a switchgear is thus relatively complex, which—in addition to the expensive gas pressure measurement equipment—further contributes to the relatively high cost of conventional switchgears.
With regard to the switchgear's potential impact on the environment and the corresponding constructive demands on the housing, efforts have been made in the past to replace the conventional insulation gases by suitable substitutes.
For example, WO 2008/073790 discloses a dielectric gaseous compound which—among other characteristics—has a boiling point in the range between about −20° C. to about −273° C., which is low-ozone-depleting, preferably non-ozone-depleting, and which has a GWP less than about 22,200. Specifically, WO 2008/073790 discloses a number of different compounds which do not fall within a generic chemical definition.
Further, EP-A-0670294 discloses the use of perfluoropropane as a dielectric gas and EP-A-1933432 refers to trifluoroiodomethane (CF3I) and its use as an insulating gas in a gas-insulated switchgear.
For improving the breakdown field strength compared to standard insulation media, U.S. Pat. No. 4,175,048 suggests a gaseous insulator comprising a compound selected from the group of perfluorocyclohexene and hexafluoroazomethane.
However, using the compounds according to the documents given above in an encapsulated switchgear requires sophisticated gas pressure measuring means, as pointed out above. Also, if high amounts of the insulation gas leak out of the housing, the reaction time for establishing sufficient insulating properties is often relatively long. In this case the panel has to be disconnected immediately to avoid damage of the switchgear.